1. Field of the Invention
The present invention relates to coaxial cables used to provide interconnection between electronic equipment and to methods for constructing a cable with a variable diameter along its length.
2. Description of Related Art
Coaxial cables are a preferred means for transmitting signals between electronic equipment. Effective data transmission between sophisticated computers and similar apparatus is dependent upon the successful utilization of such cables.
As electronic equipment speed has increased, there has been a growing demand for transmission cables that are capable of transmitting data and other signals very fast and very accurately. As such, conventional techniques such as providing more effective and consistent dielectric materials have played a crucial role in the development of better coaxial cable transmission lines. It is particularly important that such lines experience minimal signal degradation or loss along their lengths while assuring that signal integrity is maintained. Central to producing such cables has been using a substantial layer of dielectric material that has a very low dielectric constant while being very consistent in performance along its length.
A particularly preferred dielectric material comprises an expanded polytetrafluoroethylene (PTFE), such as that made in accordance with U.S. Pat. No. 3,953,566 to Gore. This material may be made into a tape or sheet form and wrapped around a conductor or may be formed as a tube that has a conductor positioned within it. Expanded PTFE has a number of important benefits over many other available dielectric materials, including lower dissipation factor, smaller cable diameter for a given impedance and conductor size, lighter weight, and faster signal speed. Despite the advantages of using an expanded PTFE material, serious design constraints still exist for those producing high speed transmission cables.
One major constraint in cable design centers around the ability to terminate many cables into a relatively small area. For example, the connection of cables into a backplane of a computer may require many cables to be fed into and connected with the computer in a very small area. With the present trends toward miniaturization and vastly higher speeds, there is a growing demand to increase the number of cable connections in an increasingly smaller connection area.
The demand for more cables connecting to a smaller area has resulted in a number of less than satisfactory compromises. Presently one of several choices is commonly employed. First, the backplane or other connection area is enlarged to accommodate the desired number of cables. This approach seriously limits the compactness of the electronic component. Second, conventional backplane designs are employed, but otherwise usable ports are not used in order to accommodate larger diameter cables. Again, this approach wastes substantial premium interface space on an electronic device. Third, smaller diameter cables are used to increase the number of cables that can be connected into a given area. Unfortunately, for any given impedance, to reduce the diameter of a cable requires reduction in the diameter of the conductor. As a result, smaller diameter cables have significantly poorer electrical characteristics that limit their use for accurate, high speed data transmission, specifically higher attenuation or loss of signal power.
A particular concern when employing smaller diameter cables is that such cables may provide inconsistent signal transmission properties. One measure of signal integrity in this regard is the "eye pattern" produced by such cables. If signal transmission is poor, the available time when multiple digital signals are "seen" at a receiver unambiguously as either "one" or "zero" is a small portion of a cycle time. If this portion becomes too small for the receiver to clearly identify the polarity of each signal in a large group, the electronic system will not operate properly and the cycle time must be increased, with reduced overall system speed.
In other areas of electronic cable design, it has been suggested to change the diameter of a cable along a long length of the cable in order to provide controlled changes in electrical performance. Although this concept may work for certain specialty applications, this process has proven hard to employ due to difficulties in mass producing cables with properly controlled changes in cable diameter. Additionally, prior to the present invention, it has never been suggested that a cable be constructed having variable cable diameters with the intent of minimizing attenuation.
Accordingly, it is a primary purpose of the present invention to provide a cable for signal transmission that provides very good electrical performance along its length while not unduly limiting the number of terminations that a group of such cables can make in a given area of space.
It is another purpose of the present invention to provide an improved method of producing a cable with variable diameters along its length.
These and other purposes of the present invention will become evident from review of the following specification.